Microelectromechanical system (MEMS) membranes are used in a spectrum of optical applications. For example, they can be coated to be reflective and then paired with a stationary mirror to form a tunable Fabry-Perot (FP) cavity/filter. They can also be used as stand-alone reflective components to define the end of a laser cavity, for example. Optical surfaces, such as curved, binary, or diffractive surfaces, can be fabricated on the membranes to create movable mirrors and lenses.
The MEMS membranes are typically produced by depositing a membrane structure over a sacrificial layer, which has been deposited on a support structure. This sacrificial layer is subsequently etched away, or otherwise removed, to produce a suspended membrane structure in a release process. Often the membrane layer is a metal or silicon compound and the sacrificial layer can be polyimide, for example.
Typically, membrane deflection is achieved by applying a voltage between the membrane and a fixed electrode on the support or an air bridge structure, for example. Electrostatic attraction moves the membrane in the direction of the fixed electrode as a function of the applied voltage. This results in changes in the reflector separation of the FP filter, cavity length in the case of a laser, or lens/mirror position.
One chronic problem associated with MEMS membranes in general is stiction. Specifically, if deflected sufficiently to contact an adjoining surface, the membranes can xe2x80x9csnap-downxe2x80x9d or adhere to that surface because of atomic-level forces, for example. One example is a suspended membrane structure that is designed to provide out of plane motion using electrostatic actuation. If the applied voltage exceeds that required to deflect the membrane to its instability point (roughly one third of the initial gap), then the membrane can snapdown. If the atomic-level bonding forces exceed the restoring force of the membrane structure, the membrane will remain xe2x80x9cstuckxe2x80x9d to the fixed electrode. Another scenario that produces a similar result is triggered by an acceleration load, when the load is sufficient to deflect the membrane to its full extent, as in a shock test.
One path to solving stiction problems includes the addition of surface features and/or coatings to the membrane, or the stationary surface adjacent the membrane, to allow the membrane to recover from a snapdown event. The contact area between the two surfaces can be reduced so that the bonding forces are reduced. Roughening the surfaces is an example of this approach as is producing discrete protrusions on either surface. A number of risks, however, are inherent with this solution. Surface roughening is not appropriate for all applications. Stiction bumps can become damaged in the event of snapdown since the electrical potential across the electrostatic cavity will be discharged through the small contact area of the bump. This can lead to bump damage or bump welding.
Another path focuses on reducing the surface energy of contacting surfaces by using a chemical treatment. Antistiction coatings, however, do not appear to be a robust solution, merely incrementally improving the survivability of membranes to snapdownxe2x80x94the coatings can also be relatively slow acting. They may also be incompatible with required optical coatings, such as dielectric antireflective (AR) coatings or highly reflective (HR) coatings for example, or damage active semiconductor devices because of organic content.
The present invention concerns the integration of tabs or stops that prevent snapdown of a deflectable membrane structure. Features comprising two surfaces separated by a distance equal to the maximum desired range of movement are produced. When the two surfaces contact, the motion of the structure is arrested or greatly diminished. For an electrostatically actuated MEMS structure, these features can be used to limit the range of motion such that pull-in or snapdown is avoided, greatly enhancing the reliability of the device. One key design feature is that the two contacting surfaces can be maintained at the same, or near the same, electrical potential avoiding problems associated with electrostatic discharge.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.